1. Field of the Invention
The invention pertains to fluorescent lamps and more particularly to fluorescent lamps having an improved dual cathode structure.
2. Description of the Prior Art
Hot cathode type fluorescent lamps ordinarily utilize a cathode/anode structure which includes a coiled tungsten wire having an emissive coating, such as barium oxide, that produces electrons when heated above 800.degree. C. The emitted electrons will bombard an ionizable medium, such as mercury, together with a fill gas inside the fluorescent lamp resulting as the ultimate step in a series of electronic interactions in the emission of ultraviolet light. A phosphor on the wall of the lamp generates visible radiation in response to the impinging ultraviolet energy.
A prime cause of failure of fluorsecent lamps is the gradual deterioration of the emissive coating on the cathode. The emissive coating, when heated evaporates off the cathode until the lamp can neither be started nor properly operated.
U.S. Pat. No. 3,369,193 describes an electrode structure for use in instant start fluorescent lamps utilizing filamentary type cathodes having only one end of the filament connected via a base contact to the power supply, and a probe or flag structure coupled to the opposite end of the filamentary cathode. The flag structure will extend into the cathode fall space so that anode current on the anode half cycle flows through the cathode.
In U.S. Pat. No. 3,898,503 assigned to the assignee of the present invention, a dual cathode structure is described utilizing a pair of electrodes at at least one end of a fluorescent lamp. The electrodes have one end connected to each other and a common junction with an included angle between the two electrodes of approximately 45 degrees. The other (outer) ends of the electrode pairs are connected to a lamp ballast. An anode flag, or probe is connected to the junction of the electrode of the cathode pair. This junction is at the apex of the included angle between the two cathodes. The anode flag or probe extends into the arc stream, that is the stream of electrons flowing between the anodes/cathodes at opposite ends of the lamp. Any anode current collected by the probe or flag passes through one or the other of the cathode pair before leaving the lamp. The cathodes are thus heated by the exiting current which tends to equalize the heat of the cathode during lamp operation which equalization in turn diffuses the "hot spot" of the cathode. In the aforementioned U.S. Pat. No. 3,898,503 it was found that the heat equalization increased the operating efficiency of the lamp as well as extending the life of the filament cathode pair.
It will be noted however, that in the above mentioned U.S Pat. No. 3,898,503 the nominal cathode voltage is applied across both the first and second cathodes. Therefore each of the cathodes must be designed to accept half the voltage. Therefore standard cathodes available and used in the industry could not be used.
The effect of the above dual cathode structure is essentially one of connecting a probe or flag in the middle of a standard cathode. Due to physical limitations, however, the cathode will be stretched out. Since ballast circuits can only supply a limited amount of current, the amount of heat produced per volume of the cathode will be decreased since the same heat will be dissipated in a larger area. Since the filament coil now runs cooler, electron emissions decrease If the wire of the coil is made smaller to produce a higher temperature for a given voltage, less emission coating can be used and consequently there will be a decrease in cathode life.
It is also known in the art to utilize a dual cathode structure without a flag comprised of first and second cathode elements coupled at a junction and forming a V having similar characteristics to that described in U.S. Pat. No. 3,898,503, except that one of the elements of the cathode pair cathode elements is shorted out and the ballast supply is coupled across the other element of the cathode pair. In such a system the shorted out cathode can only act as a cold start cathode and is not operational during the initial starting operation of the lamp. Such a cathode works as follows. As the emissive material evaporates from the running or operating cathode (i.e. the ballast coupled cathode) a hot spot formed thereon will move during the life time of the cathode filament along the length of the filament. If this hot spot is near the junction of the two cathodes the shorted cathode will be heated causing the hot spot to shift to the shorted cathode and therefore prolong lamp life.
It will be noted that in this structure, where the shorted cathode is not heated by external currents, only current captured by the middle of the cathode provides for its heating. Current captured at or near the ends of the cathode will flow directly into the power supply. If the connections to the cathode are, however, reversed and the hot spot begins at the junction of the two cathodes when the emissive material of the first cathode has been utilized, the hot spot will have moved to an end furthest away from the shorted cathode. The shorted cathode, therefore, will not be heated and therefore will not emit electrons. There is thus no enhancement of lamp life. Although the manufacture of lamps is generally standardized the manufacture of the ballast system is not and thus ballast connections may be improper. Consequently such long life fluorescent tubes only have a 25% chance that the lamp will be correctly connected and lamp life lengthened.
Generally the hot spot will begin at the end of the cathode connected to the higher potential. It will be noted that since there is a voltage drop across cathode filaments in addition to the cathode voltage, one end of the cathode filament will always have a higher potential than the other end. Generally this is where the hot spot begins.
Furthermore, in the a cold start/shorted cathode dual cathode structure refered to above the ordinarily cold or shorted cathode may become fouled by impurities boiled off from the operating cathode. These impurities often show up in lamps as a discoloration on the sidewalls of the lamp. These impurities when boiled off onto the cold cathode may cause the cold start cathode to be difficult to operate.